Problem of universals

The Problem of universals is a metaphysical concept that consists of the question of whether properties exist and, if so, what they are. The fundamental issue is the question of whether univerals exist in reality or merely in thought and speech, an issue that dated back to the time of Plato.

History
A universal is what particular things have in common, such as essences or qualities. As the result of the process of extraction, whereby we strip away whatever makes things unique, what is left is similarities to other beings; this is the way in which we comprehend universals.

The problem was especially debated during the Middle Ages, but it first arose in ancient times, and continued to be an issue even after the Middle Ages. There is a new problem that has been brought about by computer science. In metaphysics, the problem refers to the question of whether properties exist and, if so, what they are. Essences are universal, being repeatable or occuring entities that can be exemplified by certain things. There are three major kinds of properties: types/kinds (essences), properties (being short, strong, green, black, etc.), and relations (father of, next to, underneath, on top, etc.). Paradigmatically, universals are abstract, such as humanity, whereas particulars are concrete, such as the humanity of Socrates. Universals are not necessarily abstract, however, just as not all particulars are concrete. Numbers are particular, yet abstract, objects, while universals can be considered to be concrete.

A universal may have many instances, known as its particulars. Any particular object between other things is not a universal, but is an instance of a universal. While philosophers agree that humans talk and think about properties, they disagree over whether universals exist in reality, or merely in thought and speech.

Realism and Idealism
The main divides between realism and idealism/anti-realism/non-realism are major issues in the problem of universals.

Realism has a strong version (Platonic realism) and a weak version (Aristotelian realism). In Platonic realism, beauty is a property that exists in an ideal form independently of any mind or description, being self-subsistent. Aristotelianism realism would argue that beauty only exists when beautiful things exist.

In idealism, beauty is a propety created in the mind, existing only in descriptions of things. A notable variant (and, perhaps, the main form) of idealism is known as nominalism. Nominalists deny that universals exist, claiming that they are not necessary in order to explain attribute agreement. Conceptualism argues that universals exist only in the mind (when conceptualized) and deny the independent existence of universals, but arguing that existence in the mind is a real existence, differing from nominalists.

Realists claim universals are real - that is, they exist, and are distinct from the particulars that instantiate them. There are various forms, and there are two Latin expressions that typify the difference: Platonic is universalia ante res (universal before the thing), while Aristotelian is universalia in rebus (universal in the thing). Universals must be positive as distinct entities, whether subsisting in themselves or not. A common realist argument is that universals are required for certain words to have meaning, or for sentences to be true or false, such as Ludwig von Beethoven is a musician.

Nominalists say that only individuals/particulars exist, and that universals are not real (not entities or beings). They argue that entities only share a name, not a real quality, in common. There are different forms of nominalism, namely conceptualism (advocated by Peter Abelard) and nominalism (advocated by William of Ockham). Nominalists claim that nominalisn can account for all of the relative of phenomena, and William of Ockham's famous "Razor" (entities are not to be multiplied without necessity) applies to this. Whether nominalism can account for all of the relative phenomena, however, is debatable.

Platonic and Aristotelian views
Plato believed in a strong distinction between the world of perceivable objects and the world of distinctions and forms. According to Plato, one can only have opinions about perceivable objects, while one can have knowledge about forms. For Plato, knowledge regards what "is", having the marks of being permanent and unchanging; it has to be forever unfailing and general. The reason of forms in the real world, while the word of sense objects is only imperfectly, or partially, real. In Platonic realism, the eternal forms are not mere mental artifacts; in that respects, it is the polar opposite of modern idealism. Diogenes of Sinope disputed Plato's views, saying that he saw his cups and table, but not the cupness and tableness.

Aristotle transformed Plato's forms from objective essences into formal causes, comprising the intelligibility of individual things. Aristotle emphasized nature, and much of his thinking concerns living beings and their properties. He was more confident than Plato in coming to know the sensible world, being a natural scientist at heart.

Middle Ages
During the Middle Ages, the problem of universals resurfaced because of the 6th century Roman philosopher Boethius' discovery of a problem in Porphry's works. He said, "I shall omit to speak about genera and species, as to whether they subsist in the nature of things, or in mere conceptions only." He summarized the various respective positions and prophesied that it would take another treatise to solve the problem; he ultimately never got around to writing that treatise.

Medieval realism
Most medieval philosophers favored realism, while nominalism was held by relatively few philosophers. Realism was argued for by both Thomas Aquinas and Duns Scotus, with Aquinas arguing that both the essence of a thing and its existence were clearly distinct, in the case of everything except for God. In this regard, he was close to Aristotle. Scotus argued that, in a thing, there is no difference between essence and existence. He offered a different kind of distinction than that between realism and nominalism, creating the "formal distinction". In a particular thing, Scots argued, there was a plurality of distinct forms. The plurality of forms inhering in a particular thing are distinct from each other, having real existence.

Medieval nominalism
In the Middle Ages, nominalism was first formulated by Roscellinus, who lived in France from the late 11th to early 12th centuries, and was one of Peter Abelard's teachers. Nominalism's greatest exponent was William of Ockham. Abelard was the founder of conceptualism, a middle way between nominalism and realism, and Ockham went farther than Abelard, arguing that only individuals exist. Critics argue that the approaches of Abelard and William of Ockham only answer the psychological question of universals, and that conceptualism was only a moderate form of realism and was untenable.

Modern thought
In modern thought George Berkeley, David Hume, and Immanuel Kant also studied the problem. Berkeley advocated an extreme form of nominalism, disbelieving even in the possibility of a general thought as a psychological fact. He argued that, when one thinks of a triangle, it is always right-angled, obtuse, or acute, and that there is no general image of a triangle. Berkeley asserted this position due to its relation to his idealism in metaphysics, specifically to his critique of the possibility of matter. In his view, it is just as meaningless to speak of triangularity in general (aside from specific figures) as it is to speak of mass in motion without knowing the color. If the color is in the eye of the beholder, then so is the mass; reality is perception and vice versa. Forced by his extreme empiricism, Berkeley posited the existence of God to explain coherence, demonstrating the brilliance of Kant's epistemology that was to follow.

Hume tried to give such an account of squaring coherence of existence with empiricism when he proposed that concepts are the faded memories of repeated sensory experiences. This account threatened the very possibility of science as an objective endeavor, making Kant - at heart a scientist - uneasy. Hume's skepticism forced Kant to develop a theory of noumenal (unverifiable, but understandable) objects. For Kant, instead of making God the guarantor of the coherence of the world, he posited the faculty of reason, structured by the forms of intuition such as space and time and the categories of understanding (such as "cause and effect"). Kant argued that the nature of reality is based only on our minds or ideas; that our sensory experiences are filtered by the categories of our understanding. Hegel, interpreting Kant, viewed the world as inseparable from the mind, that external realities are inseparable from consciousness or perceptions. Universals were held to be real, but they existed independently.